The present invention relates to a technique effective in application to a semiconductor integrated circuit device incorporating a PLL (phase locked loop) circuit, specifically to a technique effective in use for a radio frequency semiconductor integrated circuit device incorporating a PLL circuit that generates a predetermined frequency oscillation signal synthesized with a reception signal or a transmission signal for a wireless communication, and a mobile communication system containing the same.
In a wireless communication system such as a mobile telephone, a radio frequency semiconductor integrated circuit device (hereunder, referred to as an RF IC) is used which incorporates a PLL circuit including an oscillator that generates a local oscillation signal of a predetermined frequency, being synthesized with a reception signal or a transmission signal, and modulates the transmission signal and demodulates the reception signal. Conventionally, comparably many mobile telephones have supported the so-called dual band system, which can handle signals of two frequency bands, such as a signal of the GSM (Global System for Mobile Communication) covering the band 880 MHz through 915 MHz, and a signal of the DCS (Digital Cellular System) covering the band 1710 MHz through 1785 MHz. Some of the mobile telephones supporting this dual band system are made up to handle the two bands with one PLL circuit by switching the frequency of the PLL circuit.
In recent years, however, there arises a demand for a mobile telephone of the triple band system that can handle a signal of the PCS (Personal Communication System) covering the band 1850 MHz through 1915 MHz in addition to the GSM and DCS.
These circumstances conceivably lead to a demand for mobile telephones that can handle still more systems. A voltage-controlled oscillator (VCO) being used in a mobile telephone that can handle such plural systems is required to have a wide oscillation frequency range. Here, if one VCO tries to cover all the frequencies, the sensitivity of the oscillation frequency against the control voltage of the VCO (hereunder, referred to as the control sensitivity) becomes higher, and the VCO becomes weaker to external noises and fluctuations of the power supply voltage.
In order to solve the above deficiency, a trial is made in the invention (Patent Reference 1), which switches a VCO into multiple frequency bands (16 bands) to thereby reduce the control sensitivity of the VCO, while maintaining a desired oscillation frequency range. This prior invention adopts a system to determine an optimum frequency band for use. This system measures, in advance before starting the operation, the actual frequencies on all the frequency bands of the VCO, stores the measurements in a memory, and when oscillation frequency information is given, the oscillation frequency information is compared with the frequency measurements stored in the memory to determine the optimum frequency band.
Now in the mobile telephone, in recent years have been made many efforts that incorporate as many circuits as possible into one or few semiconductor integrated circuit devices, in order to achieve downsizing of the devices and lower the cost by reducing the number of components. In an example, a trial is made to incorporate a loop filter provided on the loop of a PLL circuit contained in an RF IC into a semiconductor chip.
[Patent Reference 1] Japanese Unexamined Patent Publication No. 2003-152535
Now, the loop filter of a PLL circuit is composed of capacitive elements and resistive elements. Conventionally, the loop filter has usually been made up with externally attached elements. The reason is that an effort to attain capacitive elements having capacitances necessary for a desired characteristic on a chip ends up in confirming the necessity of an extraordinary large area for the elements on the chip. Now, to decrease the capacitances is needed to increase the resistances of the resistive elements. However, to increase the resistances will increase the thermal noises by the resistive elements. In the PLL circuit, the voltage from the loop filter is directly applied to the control terminal of the VCO. Therefore, if the thermal noises by the resistive elements constituting the loop filter are high, and the control sensitivity of the VCO is high, the thermal noises generated by the resistive elements will appear on the output of the VCO.